Planning the next
AO Silver Member
Join Date: May 2007
Location: United Kingdom
First, check the connection for the temperature sensor. If that isn't connected properly the fans will run and the warning light for the engine ECU will light up.
The temperature sensor is actually two temperature sensors in one housing on the JTS. One sensor is for the gauge ( which is working ) and the other is for the engine ECU ( which probably isn't ). If you were to connect MultiECUScan ( or similar ) to the engine ECU, you'd probably find a wildly inaccurate reading.
If the connection seems good ( remove and reseat the connector ) and there's obviously nothing wrong with the connector or wires then there may be a fault with the sensor itself.
Did the thermostat come with a new temp sensor ( I know the one I got for my wife's 156 JTS did )?
Two things you can do.
If you have a multimeter, check the resistance on pins 1 & 3 ( Brown & Blue wires ). Then check the other two pins. The two readings should be very similar. With the engine cold you're probably looking at a reading between 12,000 and 20,000 ohms. If the engine is hot the reading will be less than 10,000 ohms.
If you don't have a multimeter and everything looks good and you've tried reseating the connector, try swapping the sensor from your old thermostat to the new one.
Don't worry about the cold top hose. That will only get hot when the thermostat opens. The engine probably isn't getting hot enough if the gauge is only getting halfway between 50 & 90.